This invention relates to thermally resistant coating materials. In a more particular aspect, this invention concerns itself with high temperature coating materials based on poly (zinc phosphinates) and to a method for preparing such coatings.
The recent advent of supersonic vehicles has created a need for coating materials resistant to the high skin temperatures that are generated within the operational environment of such vehicles. Conventional protective coatings, based upon drying oils and resins such as linseed oil and alkyds, are useful only at moderately elevated temperatures. Progress has been made on extending the thermal tolerance of coatings by the development of systems utilizing organic polymers based upon condensed units with highly conjugated double bonds. Additional improvements have been made with organosilicone-based coatings. However, current temperature requirements are even higher, and coatings resistant to thermal and oxidative degradation up to 1000.degree. F in air are needed.
An attempt to provide a solution to the need for high temperature coatings indicated that coating compositions based on poly(zinc phosphinates) possess the high thermal stability required for use with supersonic vehicles. Poly(metal phosphinates) are a family of polymers based on an inorganic backbone formed by metal atoms connected by --OPO-- bridges. Because the metal centers in the poly(metal phosphinates) are surrounded by oxygen atoms, the polymer backbone is particularly resistant to oxidative degradation, and, as a rule, the polymers are very stable thermally. Among the polymer compositions found to be most stable and useful as a coating material are the metal phosphinates in which zinc is the central atom. They possess an inherent thermal stability that makes them especially useful for high temperature applications in many areas of technology.
At the present time, however, few of these materials are capable of facile conversion into coatings by standard preparative methods such as from a melt or by deposition from a solvent. Zn[OP(Me)(Ph)O].sub.2 is one of the few zinc phosphinates that are appreciably soluble in organic solvents and melt at a reasonable temperature, but the coatings obtainable to date have been unsatisfactory because of pinhole formation. Moreover, this composition is less thermally stable than others of the same type. The zinc phosphinates which show the greatest thermal stability are practically insoluble or poorly soluble in common solvents and do not melt at a reasonable point below their decomposition temperatures. Zn[OPh.sub.2 O].sub.2 is insoluble in common solvents and as a result cannot be utilized as a binder for high temperature coatings. Zn[OP(C.sub.6 H.sub.4 SO.sub.2 Ph) (Ph)O].sub.2 is practically insoluble in common solvents and required a proportionally large amount of strong coordinating solvents such as dimethyl formanide for disolution. Preparations attempted from a solution of Zn[OP(C.sub.6 H.sub.4 SO.sub.2 Ph)PhO].sub.2 in 15 parts DMF and 35 parts C.sub.6 H.sub.6 result in coatings with poor integrity and pinholes.
An attempt to overcome these problems led to the discovery that the preparation of coatings utilizing zinc phosphinates that were heretofore impossible to use because of poor solubility or intractability can be accomplished by an in-situ preparation with ingredients soluble in common solvents. Such formulations are surprisingly stable in solution and do not show any supersaturation or tendency to precipitate even though the reaction products would be expected to be insoluble.